The present invention relates to planning, monitoring and regulation of temperature gradients, or isotherms, for thermal treatment of tissue, and in particular, the cryoablation of tissue.
Thermal treatment of tissues has become an increasingly effective method of treatment for a variety of pathological cases. For example, malignancies in body organs such as liver, lungs, kidney, prostate, breast, and other organs are successfully treated by thermal ablation when heat or cold irreversibly destroys pathological tissue. Target temperatures are generally emphasized since irreversible changes that destroy a cell occur more reliably at those levels, but the time that those target temperatures are maintained are also important.
For example, consistent cell death using heat usually requires temperatures greater than 50° C. for over one (1) minute, while tissue freezing usually requires two (2) cycles of target tissue temperatures less than −20° C. for greater than three (3) minutes each, interspaced by a passive thaw of greater than three (3) minutes. These general assumptions are variable depending upon local tissue conditions of: 1) adjacent blood vessels, or vasculature, acting as heat sinks; 2) tissue perfusion from blood flow of the micro vasculature, and; 3) the overall thermal characteristics of the tissue, such as thermal conductivity and capacity, which appear related to the ratio of fluid and/or fibrous content.
In connection with cryo-treatments, the localized anatomy surrounding a target area to be treated (e.g., frozen), or tumor, may be assessed by various imaging techniques such as ultrasound (US), X-ray, computed tomography (CT) or magnetic resonance imaging (MRI).
Ultrasound clearly shows only the bright echoes from the leading edge of the ice closest to the ultrasound probe. The interface between frozen and non-frozen tissue has marked sound speed differences. Nearly all echoes are reflected with minimal signal penetrating the iceball. This causes marked shadowing behind the leading ice edge, obscuring all posterior structures.
Plain X-ray technique is only capable of showing the border of the lower density frozen tissue. This is possible, however, only if the tissue is sufficiently compressed to allow detection of the density difference as a projected shadow that is perpendicular to the main axis of the x-ray. Both ultrasound and x-ray techniques thus provide only partial information concerning the size and position of three-dimensional frozen tissue, and cannot define the full three-dimensional cryoablation volume contained within the frozen tissue.
CT and MRI techniques are preferred over ultrasound and x-ray since they can produce a reconstructed tissue volume from the usual sequence of axial images. In addition, they can be performed before, during and after intravenous contrast enhancement to assess the vascularity of the target tumor relative to the surrounding tissues. CT is more readily available than interventional MR units within the community and doesn't have metal incompatibility issues. However, MRI can provide greater contrast between the different soft and/or frozen tissues than CT and has temperature-sensitive imaging sequences. Unlike CT, MRI uses no ionizing radiation, but uses a powerful magnetic field and pulsed radiofrequency fields to cause the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. These signals can be manipulated by additional magnetic fields to build up enough information to construct a volumetric image of the target object.
U.S. Pat. Nos. 6,773,408 and 6,904,305 relate to MRI medical interventional procedures and therapies, applying ablation energy to the target tissues, and further relate to monitoring such therapy by magnetic resonance application. The technology described in the '408 and '305 patents are partially based on the fact that certain known MRI sequences are temperature sensitive, so that magnetic resonance data acquired using these procedures will indicate changes in the temperature of the tissues. For example, a magnetic resonance parameter referred to as T1 (spin-lattice relaxation time) will vary with temperature. If a magnetic resonance imaging apparatus is actuated to acquire T1 for various volume elements within the subject, the data for different elements will vary with temperature, at least within a tissue having generally the same composition. The data can be displayed as a visible image, and hence different temperatures can be shown by the differences in brightness or color within the displayed image. Unfortunately, this approach can only display the degree of the heating within a location within the body being heated.
These procedures have been well known but have not been widely adopted in the medical practice. Magnetic resonance imaging instruments include large, precise magnets which are arranged to impose a high magnetic field but also severely limit access to the target subject. Moreover, the MRI instruments must be such so as to be substantially unaffected by the MRI system's powerful magnetic field. Medical instruments constructed of non-MRI-compatible materials may be subjected to powerful undesired forces generated by magnetic interaction between the instrument and the MRI magnetic field that may distort the MRI image. Additionally, electrical circuits used within the MRI environment must be shielded because they may be subject to induced currents generated within the electrical circuitry. Induced current can lead to uncontrolled processes such as distortion of data or control signals. Electric currents induced by an external magnetic field interacting with components of electronic circuitry could have a distorting effects during its normal operation. For example, electronic circuits with switching components which are switching at high frequencies (e.g., computers) and with potential for emission of electromagnetic fields must be strictly shielded.
Another shortcoming of MRI is that commercially available MRI systems do not detect and display temperatures within frozen tissue. Research has been done evaluating ultrashort echo-times to assess the R2* parameter and generate temperature assessment within the iceball but these sequences are complex and specialized to limited centers.
What is needed is a system that provides capability of direct identification or localization of the isotherms within frozen tissue with reliable identification of the external border of the ablation volume. What is further needed is a system that is capable of providing accurate estimation of the size and position of an ablation volume in cryosurgery, since it is a goal to ablate all pathological tissue while damaging as little as possible of healthy tissue surrounding the pathological tissue. What is further needed is to enable a physician, during a procedure, to have accurate information of what tissues have been frozen. What is further needed is a system adapted to render the border of the ablation volume, or alternatively a system facilitating accurate estimation of the size and position of such a border, to decrease danger of surgical complications and avoid various deleterious consequences to the long-term health and quality of life of the recovering patient.
Attempts to provide systems for MRI-guided cryosurgery are described in U.S. Pat. No. 5,978,697 and US Pat. Application No. 2006/0155268. The '697 patent describes a system with inner and outer modules for MRI-guided cryosurgery comprising an MRI magnet having an opening for enabling access of a surgeon to the patient who is accommodated inside the MRI room, a line member of a surgical device extending through MRI magnet channel, a surgical device itself including: (a) an operating member for operating the patient; (b) a control member for controlling the operating member, the control member being positioned externally to the MRI room; (c) a line member having a first end connectable to the operating member and a second end connectable to said control member, wherein a portion of the line member is received within the channel of the MRI magnet. The line member includes an underground portion extending through an underground channel. The operating member is a cryogenic probe including a Joule-Thomson heat exchanger, and the line member is a gas tube. The control member includes a microprocessor for controlling the operation of cryosurgical device. A display member is positioned within the MRI room, the display member being electrically connected to the microprocessor for providing information relating to the operation. The cryogenic probe includes a thermal sensor electrically connected to the microprocessor.
A disadvantage of the system described in the '697 patent is the separation of control functions into inner and outer modules that requires two operators of the surgical equipment, a first operator being a surgeon positioned within the inner module, i.e., within magnetic field of the MRI equipment, and a second operator whose function includes inputting gas control commands and reporting to the surgeon the cryosurgery system status which the surgeon from his position into inner module cannot see and estimate for-himself, and cannot directly control. Another disadvantage of the system described in the '697 patent is an impossibility to display and control temperatures within ablated frozen tissue because the thermal sensor placed at the distal end of the cryoprobe can provide only information about temperature in the central point of ablation volume.
Application No. 2006/0155268 describes an MRI-guided and compatible cryosurgery system that comprises a cryoprobe operable to be cooled by expansion of high-pressure cooling gas though a Joule-Thomson orifice. This schematically presented system enable a surgeon positioned next to a patient and within an MRI magnetic environment both to monitor progress of an intervention by observing MR images of the intervention in real time, and to fully control aspects of operation of a cryosurgery. The apparatus described may remotely control a fluid supply source positioned external to the magnetic environment thereby enabling real-time MRI-guided control of a cryoablation process. A schematically presented embodiment enables calculation and display of borders of an ablation volume surrounding a cooled cryoprobe, and further enables automated control of elements of a cryoablation procedure, which elements are triggered when shape and position of the calculated ablation volume are found. The schematic intervention module may comprise a plurality of cryoprobes, an MRI-compatible template operable to guide insertion of the cryoprobe into the body of the patient, and a thermal sensor operable to be positioned at a selected position within the body. The cryoprobe may comprise a thermal sensor operable to report temperatures within the cryoprobe or to report temperature external to the cryo probe.
A shortcoming of Application No. 2006/0155268 is that complete information about temperature distribution within the frozen volume of ablation with thermal sensors is unavailable. Additionally, the described temperature distribution profile across a schematic frozen tissue formed by the tip of cryosurgical probe, to the external surface of the frozen volume, appears somewhat arbitrary and unrealistic.
Thus, there is a need to have a thermal ablation system that can provide to a physician isotherm distribution within the target tissue in order to plan, monitor and control tissue ablation using thermal response for anatomic and tissue characterization.